Method for purification of deicing fluid for recovery of glycol

ABSTRACT

In the purification of deicing fluid for recovery of glycol, initially solid particles are separated, then dissolved organic compounds and positive and negative ions are separated by an ion exchange in an ion exchanger, and finally water is separated by distillation.

TECHNICAL FIELD

The invention relates to a method and a device for purification ofdeicing fluid for recovery of glycol, especially purification of deicingfluids from airports, wherein initially solid particles are separated,then dissolved organic compounds and positive and negative ions areseparated by ion exchange in an ion exchanger, and finally water isseparated by distillation.

STATE OF THE ART

Accumulation of ice on aircraft wings is a well known problem involvinggreat risks in that the surface of the foil changes and in that theweight of the aircraft increases. At present, ice is removed from thewings of the aircraft by spraying them with so called deicing fluid,usually consisting of ethylene glycol, propylene glycol or a mixturethereof and additives of different kinds.

For economical reasons and in order to minimize the environmentalimpact, it is desirable to recover the deicing fluid to the greatestextent possible. At present, processes exist to purify and concentratethe most common deicing fluid which normally consists of pure glycol(monopropylene, monoethylene or diethylene) having additives of anionicand/or non-ionic tensides, corrosion inhibitors and, in certain cases,colourings and water.

Further development of deicing fluids has, however, resulted in thedevelopment of new types having the above mentioned deicing fluid as abasis with the addition of a thickener, the purpose of which is toaccomplish a better protection against renewed freezing of the deicingfluid due to fact that the thickener obtains a higher viscosity andtherefore remains for a longer time period on an aircraft. This is ofgreat importance on airports having very frequent traffic. The aircraftmay then have to wait for long periods of time before takeoff, and it isthen of utmost importance that the deicing fluid remains. When theaircraft takes off, the deicing fluid is sheared off.

For the time being, commercial recovery processes comprise filtration,deionization and distillation. The used deicing fluid is collected inthe deicing plant and when it enters the recovery plant it normally hasa glycol content of about 15-25% by weight in a mixture with water,solid particles, salts, etc. The initial filtration is, for instance,presently accomplished in a cotton filter having a mesh width of down toabout 5-10 μm. After filtration, the deicing fluid is passed through ionexchange filters to separate dissolved organic compounds and positiveand negative ions. Subsequently, water in the deicing fluid is separatedby distillation, possibly under vacuum, until the mixture ofglycol/water has obtained the desired concentration.

However, at airports, where deicing fluid comprising thickener is used,problems arise in connection with the recovery of glycol. The thickenercomprises a polymer, a polar organic compound, and long polymer chainsthereof block the material in the ion exchange plant. The capacity ofthe ion exchange material is then strongly reduced with extensiveoperational breakdowns as a result. The capacity of the ion exchangematerial is also reduced by anionic tensides and petroleum products,e.g. hydraulic oil, aircraft fuel etc., which, in many cases,contaminate the glycol.

A process for recovery of glycol, among other things deicing fluid, isfor instance disclosed in the Canadian Patent Application CA-A1-2116827.In this purification process fats, oils, and dirt are first separatedfrom the used glycol in a sedimentation tank having a partition wallconsisting of a metal mesh. The fluid is left to stand in thesedimentation tank until heavy solid particles have fallen down to thebottom of the tank and light organic compounds, such as fuel andhydraulic oils, float on the surface. The organic compounds are thenremoved from the surface by means of a pump. Purified glycol is removedfrom the sedimentation tank as an intermediate phase for furtherpurification in a column packed with packing material of stainlesssteel, wherein water is evaporated by means of hot air before the glycolis further purified and concentrated by vacuum distillation. Thevaporized water containing buffer, volatile organic compounds and otheradditives occurring in the deicing fluid must then be purified in aseparate process.

THE INVENTION

The object of the invention is to remove the above referenced drawbackswith the method of the invention.

The object of the invention is also to achieve a device for purificationof deicing fluid for recovery of glycol.

Due to the method and the device according to the invention, organiccompounds, such as thickener, petroleum products and anionic tensidesoccurring in the deicing fluid, may be separated from the deicing fluid.In this way, damage on the material in the ion exchangers is avoided,and, according to the invention, allowing all kinds of deicing fluids tobe purified and recovered with a higher degree of purification than hasearlier been possible to achieve. A high degree of purification of thewater separated during distillation is also achieved, said water notrequiring any extra purification but can be discharged directly to thesewage system.

SHORT DESCRIPTION OF THE DRAWING

For further explanation of the invention, an example of an embodimentthereof will be described in more detail below with reference to theappended drawing showing a flow chart of a device according to theinvention for the purification of deicing fluid for recovery of glycol.

DETAILED DESCRIPTION OF THE INVENTION

When arriving to the recycling device, the deicing fluid normally has aglycol content of 15-25 weight-% (monopropylene, monoethylene ordiethylene) with an addition of thickener, normally a maximum of 1% ofthe total amount of glycol. In addition, additives of anionic and/ornonionic tensides, corrosion inhibitors and, in certain cases,colourings in a mixture with water, solid particles, salts etc. areincluded. Since the deicing fluid normally is buffered, it has a pH ofabout 6-8.

The purification process begins with flocculation of the deicing fluidin order to obtain a formation of larger aggregates of the colloidswhich are present in the liquid, especially the polymer chains of thethickener. The tendency of the particulate phase in colloidaldispersions to form aggregates is an important physical property whichis utilized by many different separation processes, e.g. sedimentationand filtration. The formation of aggregates of colloids is denoted ascoagulation or flocculation depending on how large forces are actingbetween the colloids in the aggregate.

Particles dispersed in a fluid collide due to relative motions (Brownianmotions), and the stability of the dispersion (i.e. the stabilityagainst formation of aggregates) is determined by the interactionbetween the particles during these collisions. The forces of retractionand repulsion between the particles depend on the conditions in thefluid, e.g. salt concentration, pH etc.

Most particles achieve an electric charge on the surface when they comein contact with a polar medium. Ions having opposite charges in thefluid are attracted to the surface of the polar particle and ions havingopposite charges repel. This process, together with the motions of theparticles in the liquid, results in the formation of a diffuse electricdouble layer. In order to decrease the diffuse double layer of theparticles and, thus, decrease the distance between the colloids, aflocculating agent is added which contains ions having a charge which isopposite to the charge of the colloid surface. Due to the flocculatingagent, Van der Waals forces, which only are active at very shortdistances between the particles, may exist between the colloids in thesolution such that larger aggregates, so called flocculate, may beformed. Subsequently, the flocculate can be separated from the solutionin a subsequent separation process.

The macromolecules of the thickener included in the deicing fluidcomprise polar polymer chains which have a negative surface charge inthe deicing fluid. In order to increase the attraction forces betweenthe repelling colloids, according to the invention, differentflocculating agents have been tested, for example CaCl₂, alum andFe₂(SO₄)₃, wherein the latter has been shown to be superior regardingkinetics, efficiency, yield, etc.

In order to achieve an efficient flocculation with Fe₂(SO₄), it isessential that the amount of free Fe³⁺ ions is sufficient. Theconcentration of Fe³⁺ ions depends on the pH of the solution which alsoaffects the formation of positive or negative hydroxyl complexes anddeposition of Fe(OH)₃. The optimal amount of Fe₂(SO₄)₃ and the correctpH minimize the total content of dissolved iron in the purified liquid,which minimizes the load on the cation exchanger. An optimal pH rangefor flocculation of deicing fluid has proved to be 3.2<pH<4.8.

Referring to the FIGURE, the device for recovery according to theinvention includes a collecting tank 1 for receiving deicing fluid,collected from an airport. Subsequently, Fe₂(SO₄)₃ is added to thedeicing fluid in the collecting tank from a dosage tank 2 for Fe₂(SO₄)₃through a conduit 3 by means of a dosage pump 4 until pH for thesolution is within the range of 3.2<pH<4.8. For the measurement of thepH value in the deicing fluid, a not shown pH meter may be provided inthe collecting container 1 or samples may be taken for an analysis. Theconcentration of Fe₂(SO₄)₂ solution in the deicing fluid should be atleast 0.001 M.

As an example, it may mentioned that, in a deicing fluid having anaddition of thickener which is about 1% of the total amount of glycol,an addition of about 1.0% by volume Fe₂(SO₄)₃ having a concentration of0.1M is suitable. This results in a total concentration of Fe³⁺ ions inthe solution of about 100 ppm and a pH of about 3.2-4.8. In cases wherethe deicing fluid is strongly buffered, it may be required to add alarger amount of Fe₂(SO₄)₃ solution in order to achieve the desired pH;possibly some kind of acid may be added, for example HCl, in order tofurther decrease pH.

In order that the formation of flocculate in the deicing fluid shall beoptimal, it is important that the Fe₂(SO₄)₃ solution is thoroughly mixedwith the deicing fluid, and, for this purpose, the collecting tank 1 isprovided with an agitator 5. The kinetics for the deicing fluid furtherrequires that the flocculation is maintained for at least 2 hours inorder that the yield shall be as large as possible. In certain cases, itmay be necessary with a much longer time or flocculation, possibly up to12 hours. In the collecting tank 1, heavier particles, such as sand andgravel from the apron, sink down to the bottom at the same time as thepolymer chains of the thickener subsequently gather into largeraggregates, so called flocculate.

As an alternative to the collecting tank 1, an elongated narrowcollecting vessel may be used, wherein the deicing fluid is supplied atone end of the vessel simultaneously with the flocculating agent. Then,the fluid is slowly transported through the length of the vessel suchthat the dwell time within the vessel is at least two hours before anydeicing fluid is collected at the other end of the collecting vessel.

When the flocculation is finished, the deicing fluid is collectedthrough a conduit 6 in order to be transported to a separator 7, forexample an Alfa-Laval separator of the type used for example within thedairy industry for the separation of different phases of liquid food.The liquid is fed into the separator 7 through an inlet 8 at the bottomthereof. Since the flocculation process takes place batchwise, it may besuitable to provide several collecting tanks 1, as is shown on thedrawing, to make a continuous flow of deicing fluid from the collectingtanks 1 to a separator 7 possible.

After the centrifugation in the separator 7, remaining heavier dustparticles and the flocculated material may be collected as a heavy phasethrough an outlet conduit 9 at the bottom of the separator. Existingoils, for example hydraulic oil and diesel oil, and aircraft fuel areseparated as a light phase through an outlet conduit 10 at the top ofthe separator and are thereafter collected in an exhaust tank 11. Thedeicing fluid, now being purified from petroleum products, flocculatedmaterial comprising the thickener and other larger particles >0.5 μm, iscollected through an outlet conduit 12.

As the cation exchanger is damaged by Fe³⁺ ions, the concentrationsthereof should not exceed 10 ppm. For this purpose, a sand filter 13 isprovided which, for example, comprises silica sand having a particlesize of 0.8-1.2 mm and in which the concentration of Fe³⁺ ions isdecreased to <4 ppm. Flocculate remaining in the sand bed, whichpossibly may occur, is collected, and for this purpose it is importantthat the sand filter is provided directly after the separator. The sandfilter may, if necessary, be cleaned by so called back flashing.

After the sand filter 13, the deicing fluid is collected through aconduit 14 and, if necessary, the device may be completed with aMn-filter and/or a solids filter 15 having a mesh width of about 0.1-0.5μm. Then, the purified deicing fluid is passed through an ion exchanger16, normally comprising a humus filter (for the separation of organiccompounds), a cation exchanger (for the separation of positive ions) andan anion exchanger (for the separation of negative ions). Finally, thedeicing fluid is distilled in a not shown distillation column, whereinwater is vaporized from the purified glycol until the mixture ofglycol/water has obtained the desired concentration.

Due to the method and the device for the purification of deicing fluidaccording to the invention, the deicing fluid may pass through anordinary ion exchanger comprising a humus filter, for the separation oforganic material, and a cation and anion exchanger, for the separationof positive and negative ions, without the ion exchangers being damagedwith long term breakdowns and economical drawbacks as a consequencethereof.

What is claimed is:
 1. A method of purification of a deicing fluidcomprising glycol and a polymer thickener, and optionally anionictensides, and petroleum products for the recovery of glycol, the methodcomprsing: adding Fe₂(SO₄)₃ to the deicing fluid as a flocculatingagent; maintaining, after the adding of the Fe₂(SO₄)₃, the pH of thedeicing fluid within the range of pH 3.2-4.8; mixing the deicing fluidand the Fe₂(SO₄)₃; separating solid particles and flocculated materialincluding said polymer thickener from the deicing fluid; separatingdissolved organic compounds and positive and negative ions from thedeicing fluid via an ion exchanger; and separating water from thedeicing fluid by means of distillation.
 2. The method of claim 1,wherein the concentration of Fe₂(SO₄)₃ in the deicing fluid is at least0.001 M.
 3. The method of claim 1, wherein the adding step occurs atleast two hours before the step of separating solid particles.
 4. Themethod of claim 1, wherein the deicing fluid is separated in aseparator, wherein flocculated material and the solid particles arecollected as a heavy phase, light organic compounds are collected as alight phase and separated deicing fluid is collected as an intermediatephase.
 5. The method as in claim 4, wherein the separated deicing fluidis passed through a sand filter in order to remove remaining solidparticles and decrease the concentration of Fe³⁺ ions in the separateddeicing fluid.
 6. The method as in claim 4, wherein the separateddeicing fluid is passed through at least one of an Mn-filter and asolids filter having a mesh width of 0.1-0.5 μm.